Thursday, October 27, 2016

Latest observational data from Amundsen Sea embayment (ASE) of West Antarctica confirms the retreat of ice shelf grounding lines caused by warmer water melting the underside of the ice shelves.

Warm ocean waters are melting the junction of the ice shelves where they meet the bedrock, so that the grounding line retreats. This then allows for an acceleration in glacier dischange of ice mass to the sea increasing sea level rise.

Many of the glaciers of the West Antarctic Ice Sheet (WAIS) have topographies where they get deeper as they go further inland.

As the ice shelf grounding line retreats into deeper areas due to basal melting by warmer water, it moves much more rapidly, which then allows more warm water to continue the process of basal melting, and so the ice mass discharge from the glaciers speeds up.

Smith Glacier is an excellent example of this process. Other glaciers in this area, such as Pope and Kohler Glaciers have retreated to much shallower terrain slowing the grounding line rate of retreat and rate of ice mass discharge.

Here is how the researchers describe it:

We find melting rates in the grounding zones of the three glaciers to be much higher than steady-state levels, removing as much as 300–490 m between 2002 and 2009 in the case of Smith Glacier (40–70 m per year). The intense unbalanced melting supports the hypothesis that a large increase in ocean heat influx into the sub-ice-shelf cavities of the ASE occurred in the mid-2000s. Averaged melting rates between 2009 and 2014 are similar or lower, which also is consistent with the slower increases in mass loss observed regionally by the end of the 2000s. Nonetheless, while Pope and Kohler glaciers stabilized, the grounding line retreat of Smith persisted. We attribute the different evolution of Smith Glacier to the retreat of its grounding line deeper allowing warmer waters to flood its grounding zone, and increasing ocean thermal forcing due to the lowering of the in situ melting point; as well as to the exposure of the glacier bottom to ocean water as the grounding line retreated rapidly. In contrast, Pope and Kohler had retreated to shallower terrain. Such combinations of ocean conditions and topography sustaining high submarine melting can hasten mass loss from West Antarctica.

The studies examined three neighboring glaciers that are melting and retreating at different rates: Smith glacier, Pope glacier and Kohler glacier. They all flow into the Dotson and Crosson ice shelves in the Amundsen Sea embayment in West Antarctica, the part of the continent with the largest decline in ice.

"Our primary question is how the Amundsen Sea sector of West Antarctica will contribute to sea level rise in the future, particularly following our observations of massive changes in the area over the last two decades," said Univerrsity of California - Irvine's Bernd Scheuchl, lead author on the first of the two studies, published in the journal Geophysical Research Letters in August.

"Using satellite data, we continue to measure the evolution of the grounding line of these glaciers, which helps us determine their stability and how much mass the glacier is gaining or losing," said the Earth system scientist. "Our results show that the observed glaciers continue to lose mass and thus contribute to global sea level rise."

UCI and NASA researchers found that the Smith Glacier's grounding line had retreated 1.24 miles (2 kilometers) per year since 1996. The Pope Glacier's grounding line receded more slowly, at 0.31 miles (0.5 kilometers) annually since 1996. And the Kohler Glacier's grounding line, which had gradually retreated, actually readvanced 1.24 miles (2 kilometers) since 2011.

Previous studies using other techniques estimated the average melting rates at the bottom of the Dotson and Crosson ice shelves to be about 40 feet (12 meters) per year.

The fastest-melting glacier, Smith, lost between 984 and 1,607 feet (300 and 490 meters) in thickness between 2002 and 2009 near its grounding line, or up to 230 feet (70 meters) per year.

The new measurements are based on gauging the thickness and height of the ice via radar and laser altimetry instruments utilized in NASA's Operation IceBridge and earlier NASA airborne campaigns.

"Our observations provide a crucial piece of evidence to support that suspicion, as they directly reveal the intensity of ice melting at the bottom of the glaciers during that period," Khazendar said.

"If I had been using data from only one instrument, I wouldn't have believed what I was looking at, because the thinning was so large," he added. However, the two IceBridge instruments, which employ different techniques, both measured the same rapid ice loss.

These observations are important because it reveals the instability of the West Antarctic marine ice sheets through ocean warming.

But It's not only West Antarctica being affected by warmer ocean waters. The Totten Glacier in East Antarctica also seems to be losing ice mass and researchers suggest this is linked to ice dynamics and ocean temperature. Totten Glacier has the largest ice discharge in East Antarctica and a basin grounded mostly below sea level.

"We find that the glacier speed exceeded its balance speed in 1989–1996, slowed down by 11 ± 12% in 2000 to bring its ice flux in balance with accumulation (65 ± 4 Gt/yr), then accelerated by 18 ± 3% until 2007, and remained constant thereafter. The average ice mass loss (7 ± 2 Gt/yr) is dominated by ice dynamics (73%). Its acceleration (0.6 ± 0.3 Gt/yr2) is dominated by surface mass balance (80%). Ice velocity apparently increased when ocean temperature was warmer."

A February 2016 study assessed the Antarctic Ice Sheet mass trends for the period of 2003 to 2013. Alba Martín-Español et al (2016) found that while East Antarctica is increasing with a mass gain, far more is being lost in West Antarctica and the Antarctic Peninsula.

"The total mass loss of the AIS is −84 ± 22 Gt yr−1 over 2003–2013, with West Antarctica being the largest contributor with −112 ± 10 Gt yr−1, followed by the Antarctic Peninsula with −28 ± 7 Gt yr−1. These losses are partly compensated by a mass gain trend in East Antarctica of 56 ± 18 Gt yr−1."

A study published earlier this year by Robert M. DeConto and David Pollard in Nature titled Contribution of Antarctica to past and future sea-level rise put forward a new model explaining ice sheet and climate dynamics. This included previously underappreciated processes such as hydrofracturing of buttressing ice shelves and structural collapse of marine-terminating ice cliffs from a 2015 study (Potential Antarctic Ice Sheet retreat driven by hydrofracturing and ice cliff failure). Their model is calibrated against Pliocene and Last Interglacial sea-level estimates and applied to future greenhouse gas emission scenarios.

Deconto and Pollard concluded that "Antarctica has the potential to contribute more than a metre of sea-level rise by 2100 and more than 15 metres by 2500, if emissions continue unabated. In this case atmospheric warming will soon become the dominant driver of ice loss, but prolonged ocean warming will delay its recovery for thousands of years."

He discusses amplifying feedbacks for Greenland and Antarctica. Read a full transcript of the video below on James Hansen's blog.

"If ice sheet mass loss has a 10-year doubling time, meter-scale sea level rise would be reached in about 50 years, and multi-meter sea level rise a decade later. 20-year doubling time would require about 100 years."

"The data records are too short. But if we wait until the real world reveals itself clearly, it may be too late to avoid sea level rise of several meters and loss of all coastal cities. I doubt that we have passed a point of no return, but frankly we are not certain of that." says James Hansen.

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